Metal-to-metal and metal-to-ceramic joining are very important technological needs. Difficulties are encountered with joints for, and between non-metals and metals which are shock prone. These materials include oxide ceramics, nitride ceramics, silicon nitride, molybdenum silicide, silicon carbide and a wide variety of nitride carbides, borides, oxynitrides, boro-carbides, carbon nitrides, diamond and other common engineering metallic materials including iron, aluminum, nickel, chromium, nickel, hafnium and tungsten containing and rare earth and transition metal alloys. The usefulness of many engineering ceramics critically depends on the ability to successfully join them. One critical application is igniters for use with natural gas. Other key applications are also found in electronic devices, especially in automobiles and spacecraft.
The joining of ceramics is also considered a viable alternative technology to methods of shape-processing of large products having complex geometric components. Similarly, such ceramic joining is desired for composites of metal and non-metal materials. Designers increasingly employ ceramic-metal inserts in machining operations which require good joining processes.
As such, it is necessary that ceramics, and other non-metals, be well bonded to metal parts. Joining, as part of a manufacturing route, can offer significant advantages for the fabrication of ceramic components, whereas it is essential for fabricating ceramic-to-metal combinations.
The joining of ceramics to themselves, or to metals/composites, is a difficult process with severe requirements to obtain perfect integrity during use. Ceramics such as molysilicides, silicon carbides and Si3N4 (silicon nitride), including combinations, are required for high-temperature situations. Such are also expected to survive in corrosive environments at high-temperatures while under stress. The joints are expected to survive severe thermal, chemical and electrical gradients. The joining process should be cost effective for the overall manufacturing cost. The joining process should also not create any distortions. Brazing is a low temperature process that offers low distortion.
This application addresses well-known problems associated with the above-mentioned joining processes. Several braze alloys have been proposed. For example, with brazing temperatures of 800° C. or higher, Ag—Cu—Ti filler metals may be used, but the joints realized with these brazes can hardly survive above 400° C. in oxidizing environments even though the braze alloys are designed to melt at much higher temperatures.
The obvious way to increase the refractoriness of the joints is by using more refractory filler metals or intelligent ternary and quaternary alloys, thus escalating the manufacturing costs and undermining the materials stabilities. What would be ideal is a joining process that allows for joining at low temperatures yet yields joints that can last at much higher temperatures. However, braze alloys are expensive, often requiring the use of silver and other costly materials.
Generally, eutectic alloys are chosen to maintain a low braze temperature. Eutectic alloys including, silver-copper, Au—Pd eutectic compositions, Pd—Ni and Ni—Cr, for example, satisfy this criterion. Among these filler metals only the Ni—Cr ones can loosely be classified as active-metal brazes. Several commercial braze metals can be identified, with liquid to solid temperatures that are around 900° C. Many technically important ceramics, including silicon nitride, and carbides of silicon and molybdenum are not well wetted by conventional filler metals. Similarly, aluminides are difficult to wet. Recent developments, however, have led to a new class of brazes. These active metal brazes react chemically with the ceramic to form wettable products on their surfaces and, thus, do not require prior modification of the ceramic surface. But, the service temperatures achievable with the common active brazes that are based on Ag—Cu matrices are low. The Ni brazes with active additives, such as Cr, have been considered as refractory alternatives. Silicon nitride joints have been made via brazing with a commercial active Au—Ni—V filler metal. In general, brazing with this filler metal is not as easy and straightforward as with the Ag—Cu—Ti active braze alloys. Useful joint strength values, ±400 MPa, have been achieved, with slight improvement of the joint strength when bonding in argon environment. The use of inert environments and the use of silver make brazing operation costly. Alternatives are required.